IT-CCD and manufacturing method thereof

ABSTRACT

The present invention provides a power-thrifty IT-CCD having a charge transfer electrode area thinned for improving the light reception efficiency of a photoelectric conversion section and being capable of executing high-speed and high-sensitivity transfer without lowering withstand voltage between charge transfer electrodes. A first insulation film is formed on the surface of a silicon substrate, and inter-electrode insulation films made of silicon oxide films and charge transfer electrodes made of polycrystalline silicon films are formed on the surface of the first insulation film. The inter-electrode insulation films are formed from side walls of the polycrystalline silicon films.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an IT-CCD and a manufacturing methodthereof and in particular to an IT-CCD and a manufacturing methodthereof. Especially, the invention relates to an IT-CCD intended forminiaturization and high quality of an inter-electrode insulation filmof charge transfer electrodes of a single-layer structure.

[0003] 2. Description of the Related Art

[0004] An IT-CCD used with an area sensor, etc., has charge transferelectrodes for transferring signal charges from a photoelectricconversion section. The charge transfer electrodes are placedcontiguously on a charge transfer passage formed on a semiconductorsubstrate and driven in order.

[0005] The IT-CCD having charge transfer electrodes involves thefollowing problem: With the advance of miniaturization according to thedemand for increasing the number of photograph pixels, the area in whichincidence light from a slanting direction can be gathered lessens andthe sensitivity is degraded. Thus, it is required that any area otherthan a photoelectric conversion section be thinned as much as possible.Since a light shield film needs to be formed in any area other than thephotoelectric conversion section, preferably the area is flattened asmuch as possible.

[0006] Thus, an IT-CCD having single-layer wiring electrodes with chargetransfer electrodes placed without being superposed is proposed. TheIT-CCD has an inter-electrode insulation film between contiguous chargetransfer electrodes, and a silicon-based conductive material such aspolycrystalline silicon is used as the material of the charge transferelectrodes.

[0007] On the other hand, the IT-CCD requires high-speed transfer ofsignal charges with upsizing and an increase in the number of pixels. Todrive a charge transfer electrode of a single-layer structure with ahigh-speed pulse, the distance (gap) from the contiguous charge transferelectrode needs to be formed narrow (0.1 μm or less). High withstandvoltage is required for insulation between electrodes.

[0008] To provide such an electrode pattern, for example, an EB directdrawing method is used on a flat surface; an expensive stepper needs tobe used. Even if an electrode pattern can be provided, it is extremelydifficult to fill a minute inter-electrode area with an insulating film,also causing withstand voltage to be lowered; it is practicallyimpossible.

[0009] It is therefore an object of the invention to provide apower-thrifty IT-CCD having a charge transfer electrode area thinned forimproving the light reception efficiency of a photoelectric conversionsection and being capable of executing high-speed and high-sensitivitytransfer without lowering withstand voltage between charge transferelectrodes. It is another object of the invention to provide amanufacturing method of an easily manufactured and highly reliableIT-CCD.

SUMMARY OF THE INVENTION

[0010] To the end, according to one aspect of the invention, there isprovided an IT-CCD including a plurality of charge transfer electrodesformed on an insulation film on a surface of a semiconductor substrate,wherein the contiguous charge transfer electrodes are separated by aninter-electrode insulation film and are placed so that they are notsuperposed on each other, and wherein the inter-electrode insulationfilm is an insulation film formed from a side wall of one of thecontiguous charge transfer electrodes.

[0011] This configuration would make it possible to provide apower-thrifty IT-CCD having a charge transfer electrode area thinned forimproving the light reception efficiency of a photoelectric conversionsection and being capable of executing high-speed and high-sensitivitytransfer without lowering withstand voltage between charge transferelectrodes. The surface can also be flattened and to form a wiringstructure on the top face, efficient pattern formation is also madepossible.

[0012] In the IT-CCD of the invention, the spacing between the chargetransfer electrodes formed by the inter-electrode insulation film is 0.1μm or less. If the spacing between the charge transfer electrodes is 0.1μm or less, it is extremely difficult to fill the space between theelectrodes with an insulating film; however, the inter-electrodeinsulation film is formed from the side wall of one of the contiguouscharge transfer electrodes, so that the spacing between the chargetransfer electrodes can be made 0.1 μm or less. Therefore, alow-resistance and highly reliable IT-CCD that can also be driven with ahigh-speed pulse can be provided.

[0013] In the IT-CCD of the invention, the charge transfer electrodesare formed so as to become the same surface as the upper end of theinter-electrode insulation film. In doing so, electrode conductor can befilled at the maximum and resistance can be lowered and in addition, thesurface can be flattened.

[0014] In the IT-CCD of the invention, the charge transfer electrodecontains a conductive film made of a silicon-based material.

[0015] In the IT-CCD of the invention, the charge transfer electrode isa multi-layer structure film of a silicon-based conductive film made ofa silicon-based material and a conductive film containing metal formedon the silicon-based conductive film. In doing so, it is made possibleto make lower resistance of the electrode.

[0016] In the IT-CCD of the invention, the conductive film containstungsten. In doing so, resistance can be lowered and a light shieldfunction can be provided by tungsten; it is made possible to provide ahighly reliable IT-CCD at low costs.

[0017] According to another aspect of the invention, there is provided amanufacturing method of an IT-CCD including a plurality of chargetransfer electrodes formed on a first insulation film on a surface of asemiconductor substrate, the manufacturing method including a firstconductive film formation step of forming a first conductive filmforming at least a part of the charge transfer electrode on the firstinsulation film; a step of forming an etching stopper layer made of amaterial having etching selectivity for a material forming the firstconductive film on the first conductive film; a step of patterning thefirst conductive film and the etching stopper layer by photolithographyto form a two-layer structure pattern of the first conductive film andthe etching stopper layer; a step of forming a second insulation film onthe whole substrate surface so as to cover the two-layer structurepattern; a side wall insulation film formation step of anisotropicallyetching the second insulation film in a vertical direction so as toleave the second insulation film only on side walls of the two-layerstructure pattern; a second conductive film formation step of forming onthe insulation film a second conductive film forming at least a part ofthe charge transfer electrode so as to cover the whole two-layerstructure pattern until the surface becomes flat; an etching-back stepof etching back the second conductive film until the etching stopperlayer is exposed; and an etching stopper layer removal step of removingthe etching stopper layer.

[0018] According to the method, anisotropic etching is used to forminter-electrode insulation film on the side walls of the charge transferelectrodes formed at intervals of two, so that a minute and highlyreliable IT-CCD can be easily formed. The inter-electrode insulationfilm is formed in a self-alignment manner as side wall insulation filmrather than formed directly by thermal oxidation, and thus can be formedat low temperature and in addition, need not be buried in a pattern of aminute width or a minute groove exceeding the resolution limit.

[0019] In the manufacturing method of the invention, the side wallinsulation film formation step includes a step of performing anisotropicetching with the first insulation film as an etching stopper. Thus,anisotropic etching is performed with the first insulation film on thesubstrate surface as an etching stopper, so that a decrease in thethickness of the first insulation film is prevented, making it possibleto prevent degradation of the withstand voltage characteristic.

[0020] In the manufacturing method of the invention, the first or secondconductive film uses a polycrystalline silicon film.

[0021] In the manufacturing method of the invention, the etching-backstep is executed by chemical polishing (CMP).

[0022] In the manufacturing method of the invention, the secondinsulation film uses a silicon oxide film. Since the silicon oxide filmdoes not contain conductive impurities, withstand voltage can be raisedeven in small thickness.

[0023] The manufacturing method of the invention further includes anetching step of etching surfaces of the first and second conductivefilms to a position lower than the upper end of the side wall insulationfilm; a metal film formation step of forming a metal film on the wholesurface; and a flatting step of etching back the metal film until thetop face of the side wall insulation film is exposed and flatting thesurface after the etching stopper layer removal step.

[0024] To form the conductive film of a polycrystalline silicon film,etc., it is difficult to decrease the resistance value, but the methodenables a metal film to be easily deposited on the top layer, making itpossible to form low-resistance and highly reliable charge transferelectrodes.

[0025] Thus, according to the invention, using anisotropic etching, theinsulation films formed on the side walls of the first-layer chargetransfer electrodes are made the inter-electrode insulation films andsecond-layer charge transfer electrode is formed therebetween, so thatthe surface can be flattened, the height of the device can be decreased,the working margins in the photolithography and etching steps arewidened, and it is made possible to provide IT-CCDs at a high yieldwithout using semiconductor manufacturing equipment such as an expensivestepper. Since the high-quality insulation films are used as theinter-electrode insulation films, the withstand voltage can be improvedand the yield can be enhanced. Further, insulation material need not beburied in the inter-electrode area of a minute width, lowering of thewithstand voltage can be prevented, and it is made possible to enhancethe yield.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the accompanying drawings:

[0027]FIGS. 1A and 1B are drawings to show a schematic configuration ofan IT-CCD of a first embodiment of the invention;

[0028]FIGS. 2A to 2F are drawings to show a manufacturing process of theIT-CCD of the first embodiment of the invention;

[0029]FIG. 3 is a drawing to show a schematic configuration of an IT-CCDof a second embodiment of the invention;

[0030]FIGS. 4A to 4H are drawings to show a manufacturing process of theIT-CCD of the second embodiment of the invention; and

[0031]FIGS. 5A to 5F are drawings to show a manufacturing process of anIT-CCD of a fourth embodiment of the invention.

[0032] In the drawings, the reference numeral 1 to a silicon substrate;2 to a first insulation film (gate insulation film); 3 a to a secondinsulation film (silicon oxide film); 3 to an inter-electrode insulationfilm; each of 4 a and 4 b to a polycrystalline silicon film; each of 5a, 5 b, and 5 to a Tungsten film; 6 to a silicon oxide film; 30 to aphotodiode; 31 to a channel stop area; 40 to a charge transferelectrode; and 50 to a light shield film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Referring now to the accompanying drawings, there are shownpreferred embodiments of the invention.

[0034] (First Embodiment)

[0035]FIGS. 1A and 1B shows a schematic configuration of an IT-CCD of afirst embodiment of the invention. FIG. 1A is a schematic plan view toshow charge transfer electrodes and other parts of the IT-CCD, and FIG.1B is a sectional view taken on line A-A in FIG. 1B. As shown in FIGS.1A and 1B, in a silicon substrate 1, a plurality of photodiodes 30 areformed and charge transfer electrodes 40 for transferring signal chargesdetected by the photodiodes are formed so as to show a meandering shapeamong the photodiodes 30. Charge transfer channels (not shown) where thesignal charges transferred by the charge transfer electrodes 40 move arealso formed so as to show a meandering shape in a direction crossing thedirection in which the charge transfer electrodes 40 are extended. InFIG. 1A, inter-electrode insulation films 3 formed in the proximity ofthe boundary between the photodiode area and the charge transferelectrodes 40 are not shown.

[0036] As shown in FIG. 1B, the photodiodes 30, the charge transferelectrodes (not shown), channel stop areas 31, and charge read areas(not shown) are formed in the silicon substrate 1, and insulation films(gate insulation films) 2 are formed on the surface of the siliconsubstrate 1. Inter-electrode insulation films 3 made of silicon oxidefilms and the charge transfer electrodes 40 made of polycrystallinesilicon films 4 a and 4 b are formed on the surface of the gateinsulation film 2. The inter-electrode insulation film 3 is formed froma side wall of either of the polycrystalline silicon films 4 a and 4 b,for example, the polycrystalline silicon film 4 a.

[0037] In an upper portion of the IT-CCD, a light shield film 50 isplaced except for the photodiode 30 portion and further a color filter60 and a microlens (not shown) are placed. The space between the chargetransfer electrode 40 and the light shield film 50 and the space betweenthe light shield film 50 and the color filter 60 are filled with aninsulation substance. The parts except the charge transfer electrodes 40or the inter-electrode insulation films 3 are similar to those in arelated art and therefore will not be discussed. FIGS. 1A and 1B showthe IT-CCD of a honeycomb structure, but the invention can also beapplied to an IT-CCD of an interline type, needless to say.

[0038] Next, a manufacturing process of the IT-CCD will be discussedwith FIGS. 2A to 2F. To begin with, as shown in FIG. 2A, a silicon oxidefilm 15 nm thick, a silicon nitride film 50 nm thick, and a siliconoxide film 10 nm thick are formed on the surface of an n-type siliconsubstrate 1 to form a gate insulation film 2 of a three-layer structure.Subsequently, a low-pressure CVD method using a gas mixture of SiH₄ andPH₃ diluted with He is used to form a polycrystalline silicon film 4 adoped at high concentration 0.4 μm thick on the gate insulation film 2.The substrate temperature at this time is 600° C. to 700° C.Subsequently, a silicon oxide film 6 used as an etching stopper layer isformed using low-pressure CVD method, and a coat of a resist called THMRmanufactured by Tokyou Ouka is applied in thickness of 0.8 to 1.4 μmonto the silicon oxide film 6.

[0039] Using photolithography, with any desired mask, exposure to lightis executed and developing and water washing are performed to form aresist pattern R having a pattern width of 0.5 μm. At this time, theresolution limit was 0.5 μm.

[0040] After this, as shown in FIG. 2B, the polycrystalline silicon film4 a and the silicon oxide film 6 are selectively etched and removed withthe resist pattern R as a mask and with the gate insulation film 2 as anetching stopper by reactive ion etching using a gas mixture of HBr andO₂ and then the resist pattern R is stripped off. Here, it is desirablethat an etching apparatus of ECR, ICP, or the like should be used.

[0041] Subsequently, as shown in FIG. 2C, a low-pressure CVD methodusing a gas mixture of TEOS and O₂ is used to form a second insulationfilm 3 a made of a silicon oxide film 30 nm thick.

[0042] As shown in FIG. 2D, etching is advanced only in the verticaldirection as anisotropic etching and etching is performed so as to leavethe second insulation film (silicon oxide film) 3 a only on side wallsof the polycrystalline silicon film 4 a to form inter-electrodeinsulation films 3 made of side wall insulation films.

[0043] Next, as shown in FIG. 2E, a low-pressure CVD method using a gasmixture of SiH₄ and PH₃ is used to form a polycrystalline silicon film 4b doped at high concentration 0.4 to 1.4 μm thick.

[0044] As shown in FIG. 2F, the substrate surface is polished by CMP(chemical polishing) and further chemical etching is performed until thetop faces of the inter-electrode insulation films 3 are exposed, andcharge transfer electrodes made up of the polycrystalline silicon films4 a and 4 b are separated. An insulation film, a light shield film, andthe like are formed on the top layer to provide an IT-CCD as shown inFIG. 1.

[0045] According to the method, when the insulation film pattern as theinter-electrode insulation films is formed, minute and highly reliableinter-electrode insulation films are easily formed by leaving the sidewalls using the anisotropic etching. Therefore, it is made possible toform an IT-CCD having minute inter-electrode insulation films smallerthan the resolution limit.

[0046] (Second Embodiment)

[0047] Next, a second embodiment of the invention will be discussed. Inthe first embodiment, the conductive layer of the charge transferelectrode is formed of one layer of polycrystalline silicon film; in thesecond embodiment, a two-layer structure formed with a conductive filmcontaining metal on the surface is adopted to make low resistance ofcharge transfer electrode.

[0048]FIG. 3 shows a schematic configuration of an IT-CCD of the secondembodiment of the invention. It is a sectional view corresponding toFIG. 1A. Conductive films of tungsten films 5, etc., are deposited onpolycrystalline silicon films 4 a and 4 b. As the conductive films,tungsten silicide, tantalum, titanium, molybdenum, nickel, silicidethereof, aluminum, etc., may be used. Other parts are similar to thoseof the IT-CCD previously described with reference to FIG. 1 andtherefore will not be discussed again.

[0049]FIGS. 4A to 4H are drawings to show a manufacturing process of theIT-CCD of the second embodiment. FIGS. 4A to 4E are similar to FIGS. 2Ato 2E to show the manufacturing process of the IT-CCD of the firstembodiment and therefore will not be discussed again.

[0050] As shown in FIG. 4F, surfaces of polycrystalline silicon films 4a and 4 b are etched and removed by reactive ion etching using a gasmixture of HBr and O₂ and the tops of inter-electrode insulation films 3are exposed until positions lower than the upper end face. As shown inFIG. 4G, a low-pressure CVD method using WF₆ and H₂ is used to form atungsten film 5 500 to 600 nm thick on the polycrystalline silicon films4 a and 4 b. The substrate temperature at this time was 500° C. At thistime, the substrate surface is flat without asperities.

[0051] Subsequently, as shown in FIG. 4H, reactive ion etching using agas mixture of CF₄ and O₂ is performed so that the top faces of theinter-electrode insulation films 3 are exposed and the substrate surfacebecomes flat, and charge transfer electrodes made up of thepolycrystalline silicon films 4 a and 4 b and the tungsten film 5 areseparated. An insulation film, a light shield film, and the like areformed on the top layer to provide an IT-CCD as shown in FIG. 3. In thiscase, the tungsten film 5 has a thickness to the extent to which itprovides a sufficient light shield effect, whereby the light shield filmcan also be omitted.

[0052] (Third Embodiment)

[0053] Next, a third embodiment of the invention will be discussed. Inthe first and second embodiments, the silicon oxide film 6 is used asthe etching stopper layer for masking or etching-back to pattern thepolycrystalline silicon films 4 a, but the etching stopper layer is notlimited to the silicon oxide film and may be a silicon nitride film or ametal film such as a chromium thin film. It may be any if it is amaterial having etching selectivity with the polycrystalline siliconfilm of the lower layer and the polycrystalline silicon film of theupper layer.

[0054] (Fourth Embodiment)

[0055] Next, a fourth embodiment of the invention will be discussed. Inthe second embodiment, the surfaces of the polycrystalline silicon films4 a and 4 b are removed to the positions lower than the upper end faceof the inter-electrode insulation film 3 and then the tungsten film 5 isformed on the polycrystalline silicon films 4 a and 4 b; in the fourthembodiment, polycrystalline silicon films 4 a and 4 b are formed andthen tungsten films 5 a and 5 b are formed.

[0056] Next, a manufacturing process of an IT-CCD of the fourthembodiment will be discussed with FIGS. 5A to SF. To begin with, asshown in FIG. 5A, a silicon oxide film 15 nm thick, a silicon nitridefilm 50 nm thick, and a silicon oxide film 10 nm thick are formed on thesurface of an n-type silicon substrate 1 to form a gate insulation film2 of a three-layer structure. Subsequently, a low-pressure CVD methodusing a gas mixture of SiH₄ and PH₃ is used to form a polycrystallinesilicon film 4 a doped at high concentration 0.4 μm thick on the gateinsulation film 2 and further a CVD method using WF₆ is used to form atungsten film 5 a. The substrate temperature at this time is 500° C.

[0057] Subsequently, a silicon oxide film 6 is formed using low-pressureCVD method, and a coat of a resist called FDUR manufactured by TokyouOuka is applied in thickness of 0.8 to 1.4 μm onto the silicon oxidefilm 6. Using photolithography, with any desired mask, exposure to lightis executed and developing and water washing are performed to form aresist pattern R having a pattern width of 0.35 μm. At this time, theresolution limit was 0.35 μm.

[0058] After this, as shown in FIG. 5B, the silicon oxide film 6 and thetungsten film 5 a are patterned with the resist pattern R as a mask byreactive ion etching using a gas mixture of Cl₂ and O₂ and then thepolycrystalline silicon film 4 a is selectively etched and removed withthe gate insulation film 2 as an etching stopper and further the resistpattern R is stripped off. Here, it is desirable that an etchingapparatus of ECR, ICP, or the like should be used.

[0059] Subsequently, as shown in FIG. 5C, a low-pressure CVD methodusing a gas mixture of TEOS and O₂ is used to form a third insulationfilm 3 a made of a silicon oxide film 30 nm thick.

[0060] As shown in FIG. 5D, etching is advanced only in the verticaldirection as anisotropic etching and etching is performed so as to leavethe third insulation film 3 a only on side walls of the polycrystallinesilicon film 4 a and the tungsten film 5 a to form inter-electrodeinsulation films 3 made of side wall insulation films.

[0061] Subsequently, as shown in FIG. 5E, a CVD method using a gasmixture of SiH₄ and PH₃ is used to form a polycrystalline silicon film 4b doped at high concentration 0.3 μm thick and further a CVD methodusing WF₆ is used to form a tungsten film 5 b. As shown in FIG. 5F,anisotropic etching is performed until the second insulation film 6 isexposed by etching-back.

[0062] A tungsten silicide film may be used in place of the tungstenfilm 5 a, 5 b in the fourth embodiment. Tantalum, titanium, molybdenum,nickel, silicide thereof, aluminum, etc., can also be used.

[0063] As described above, according to the invention, it is madepossible to provide a power-thrifty IT-CCD having a charge transferelectrode area thinned for improving the light reception efficiency of aphotoelectric conversion section and being capable of executinghigh-speed and high-sensitivity transfer without lowering withstandvoltage between charge transfer electrodes. As resistance of the chargetransfer electrodes is lowered, the electrode height can be furthermorelowered and the surface can be flattened, so that it is made possible todecrease optical characteristic failure caused by level difference, suchas inconsistencies in color. Further, since high-speed transfer is madepossible, optical characteristics of smear, etc., can be improved and itis made possible to provide a high-quality and highly reliable CCD.

[0064] According to the manufacturing method of the IT-CCD of theinvention, highly reliable IT-CCDs can be manufactured easily.

What is claimed is:
 1. An IT-CCD comprising: a plurality of chargetransfer electrodes formed on an insulation film on a surface of asemiconductor substrate; wherein the contiguous charge transferelectrodes are separated by an inter-electrode insulation film and areplaced so that they are not superposed on each other, and wherein theinter-electrode insulation film is an insulation film formed from a sidewall of one of the contiguous charge transfer electrodes.
 2. The IT-CCDaccording to claim 1, wherein spacing between the charge transferelectrodes formed by the inter-electrode insulation film is 0.1 μm orless.
 3. The IT-CCD according to claim 1, wherein the charge transferelectrodes are formed so as to become the same surface as the upper endof the inter-electrode insulation film.
 4. The IT-CCD according toclaims 1, wherein the charge transfer electrode contains a conductivefilm made of a silicon-based material.
 5. The IT-CCD according to claim4, wherein the charge transfer electrode is a multi-layer structure filmof a silicon-based conductive film made of a silicon-based material anda conductive film containing metal formed on the silicon-basedconductive film.
 6. The IT-CCD according to claim 4, wherein theconductive film contains tungsten.
 7. A manufacturing method of anIT-CCD including a plurality of charge transfer electrodes formed on afirst insulation film on a surface of a semiconductor substrate, saidmanufacturing method comprising the steps of: first conductive filmforming for forming a first conductive film which forms at least a partof the charge transfer electrode on the first insulation film; etchingstopper layer forming for forming an etching stopper layer made of amaterial having etching selectivity for a material forming the firstconductive film on the first conductive film; two-layer patterning forpatterning the first conductive film and the etching stopper layer byphotolithography to form a two-layer structure pattern of the firstconductive film and the etching stopper layer; second insulation filmforming for forming a second insulation film on the whole substratesurface so as to cover the two-layer structure pattern; side wallinsulation film forming for anisotropic etching the second insulationfilm in a vertical direction so as to leave the second insulation filmonly on side walls of the two-layer structure pattern; second conductivefilm forming for forming on the insulation film a second conductive filmforming at least a part of the charge transfer electrode so as to coverthe whole two-layer structure pattern until the surface becomes flat;etching-back for etching back the second conductive film until theetching stopper layer is exposed; and etching stopper layer removing forremoving the etching stopper layer.
 8. The manufacturing methodaccording to claim 7, wherein said step of side wall insulation filmforming includes a step of performing anisotropic etching with the firstinsulation film as an etching stopper.
 9. The manufacturing methodaccording to claim 7, wherein the first or second conductive film is apolycrystalline silicon film.
 10. The manufacturing method according toclaim 7, wherein said etching-back step is executed by CMP.
 11. Themanufacturing method according to claim 7, wherein the second insulationfilm is a silicon oxide film.
 12. The manufacturing method according toclaim 7, further comprising the steps of: etching for etching surfacesof the first and second conductive films to a position lower than theupper end of the side wall insulation film; metal film forming forforming a metal film on the whole surface; and flatting for etching backthe metal film until the top face of the side wall insulation film isexposed and flatting the surface after the step of etching stopper layerremoving.